1. Field of the Invention
This invention relates to an apparatus and a method for pulling up single crystals and a single crystal clamping device used for the same, which device is installed in equipment used for fabricating single crystals by the CZ method. This invention especially relates to an apparatus suitable for fabricating heavy single-crystal bodies.
2. Description of the Related Art
Single crystals are conventionally fabricated by the CZ method. In the CZ method, polycrystalline silicon lumps are fed into a quartz crucible of a single-crystal fabricating device, then the quartz crucible is heated by heaters disposed therearound to melt the polycrystalline silicon lumps, thereafter a seed crystal installed on a seed chuck is dipped into the melt. After that, the seed chuck and the quartz crucible are respectively driven to rotate in opposite or identical directions, and at the same time the seed chuck is pulled up to grow a single-crystal silicon ingot of predetermined diameter and length.
In the seed crystal, a dislocation will arise due to the heat shock occurring at the time of dipping it into the melt. In order to stop the propagation of a dislocation from the seed crystal to the single crystal to be grown, a dash-neck method is used. According to the dash-neck method, a reduced portion with a diameter of several mm is grown up beneath the seed crystal so as to release the dislocation to the surface of the reduced portion. After making certain that the propagation of dislocation has been stopped, a shoulder is formed and the single crystal is enlarged to comply with a predetermined diameter. Then, the pulling up operation is shifted from the enlarging step to forming the straight body (hereinafter referred to as "body") of the single crystal.
In recent years, following the enlargement of single crystals, the weight of an individual single crystal is increasing, and the strength extent of the reduced portion is reaching its limit. Therefore, in conventional processes for pulling up crystals, there exists a danger of breaking the necked portion, and single crystals could not be safely pulled up. To solve the problem, measures are taken to clamp a single crystal by engaging a clamping member at a reduced portion that is formed after the propagation of dislocation has been stopped by the reduction process in the dash-neck method. Due to the fact the most of the weight of the single crystal is held by the clamping member, breaking of the reduced portion can be prevented. Even if the reduced portion breaks, dropping of the single crystal can also be avoided.
However, when the single crystal clamping device is suspended by a plurality of wires and the wires are wound up by driving a winding drum, it is difficult to precisely control the wound-up length of each winding up drum. Accordingly, the single crystal clamping device becomes inclined, and the weight of the single crystal can not be equally loaded between the wires.
Furthermore, this deviation induces the eccentric rotation of the single crystal clamping device and causes the vibration of the clamping device, and thus dislocates the growing single crystal held therein. Furthermore, as the single crystal clamping device is suspended from the top end of the single crystal fabricating apparatus by wires, the center of the single crystal clamping device is not congruous with the center of the growing single crystals. Accordingly, the clamping can not be firmly achieved.